The isolation of individual colonies of micro-organisms is a procedure that is important for many reasons in scientific endeavour, and is specifically important for identifying micro-organisms for medical diagnostic purposes. It is used for a number of different micro-organisms but principally bacteria.
Generally isolation of single colonies is performed manually by laboratory technicians who utilise a metal or plastics loop to make multiple streaks of an inoculum on a first band in a first direction of an agar plate and produce a second band of multiple streaks with a fresh or freshly sterilised loop crossing the first band, and typically also a third and fourth band. Typically, a non-oxidising metal (platinum or nichrome) loop is held within a holder and is sterilised by heating either in the flame of a bunsen burner or in an electric heater, the loop being rapidly cooled in air or on the edge of the agar based, solid medium being used before picking up the inoculum either from the first, second or third band or other source of the microorganism. Alternatively the technician may utilise disposable loops in a similar manner as described above.
One drawback is that the work is relatively tedious and requires employment of a skilled person to perform often hundreds of isolations at a time. This is costly and the highly repetitious nature of the work can lead to errors which may have severe consequences in a diagnostic setting.
There have been several suggestions for the automation of streaking for isolated colonies. Some of the suggestions such as in patent specifications GB 2025457, EP 073774, U.S. Pat. No. 3,962,040 and U.S. Pat. No. 3,623,958 use dilution spiralling or oscillation movement of a head connected by a tube to a container holding a suspension of microorganisms. These suggestions are not suitable for readily isolating colonies from large numbers of samples. Other suggestions are more technically advanced and disclose automated isolation systems for multiple samples with the capacity for streaking samples onto a range of different petri plates for differential testing, and include delivery mechanisms for retrieval of plates from storage and placing plates on a streaking station, examples of these are disclosed in patent specifications U.S. Pat. No. 4,981,802, U.S. Pat. No. 4,287,301, and U.S. Pat. No. 6,617,146.
None of these, however, particularly address the efficiency of the mechanism of spreading to produce single, isolated microbial colonies, all use a head with a narrow contact surface requiring multiple drawing motions, each representing a streak for each band. There have been suggestions for more efficient devices for streaking micro-organisms and in particular the provision of a distinctly separate surface that can sequentially be used to present a separate sterile portion to streak the first band, the second band and finally the third band. Two such disclosures namely patent specifications U.S. Pat. No. 4,010,077 and U.S. Pat. No. 4,687,746, whilst alleviating the need to sterilise the device between streaking separate bands, still require repetitive streaking for each band.
U.S. Pat. No. 4,102,748 discloses a spreader tool providing three relatively elongated surfaces for spreading three bands on a solid growth surface. Such a tool is however, not ideal for all types of inocula. Thus, where a liquid depot of the sample is provided, the use of this spreader tool will tend to carry with it a large quantity of the inoculum, as distinct to the multiple streaks formed by a loop where only a small sample of the liquid is carried, and thus the U.S. Pat. No. 4,102,748 tool in some samples may not give adequate dilution within the confines of a petri plate to obtain isolated colonies.
One of the problems associated with a wide head surface is that solid media is not always formed perfectly smooth or precisely level with the petri plate and therefore contact with a head in an automated system needs to have provision to accommodate such variation. The other major disadvantage of the device disclosed in U.S. Pat. No. 4,102,748 is that the device is designed for manual operation and there is no indication of how this device might be adapted to an high volume automated system.